Butadiene is an important base chemical and is used, for example, for production of synthetic rubbers (butadiene homopolymers, styrene-butadiene rubber or nitrile rubber) or for production of thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is also converted to sulfolane, chloroprene and 1,4-hexamethytenediamine (via 1,4-dichlorobutene and adiponitrile). Through dimerization of butadiene, it is also possible to obtain vinylcyclohexene, which can be dehydrogenated to styrene.
Butadiene (1,3-butadiene) can be prepared by thermal cracking (steamcracking) of saturated hydrocarbons, typically proceeding from naphtha as the raw material. The steamcracking of naphtha affords a hydrocarbon mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butanes, butenes, 1,2-butadiene and 1,3-butadiene, butynes, methylallene, and C5 and higher hydrocarbons.
Butadiene can also be obtained by the oxidative dehydrogenation of n-butenes (1-butene and/or 2-butene). The input gas utilized for the oxidative dehydrogenation (oxydehydrogenation, ODH) of n-butenes to butadiene may be any desired mixture comprising n-butenes. For example, it is possible to use a fraction which comprises n-butenes (1-butene and/or 2-butene) as the main constituent and has been obtained from the C4 fraction from a naphtha cracker by removing butadiene and isobutene. In addition, it is also possible to use gas mixtures which comprise 1-butene, cis-2-butene, trans-2-butene or mixtures thereof and have been obtained by dimerization of ethylene as input gas. In addition, input gases used may be gas mixtures which comprise n-butenes and have been obtained by catalytic fluidized bed cracking (fluid catalytic cracking, FCC).
Processes for oxidative dehydrogenation of butenes to butadiene are known in principle.
US 2012/0130137 A1, for example, describes a process of this kind using catalysts comprising oxides of molybdenum, bismuth and generally further metals. For the lasting activity of such catalysts for the oxidative dehydrogenation, a critical minimum level of partial oxygen pressure is required in the gas atmosphere in order to avoid an excessive reduction and hence a loss of performance of the catalysts. For this reason, it is generally also not possible to work with a stoichiometric oxygen input or complete oxygen conversion in the oxydehydrogenation reactor (ODH reactor). US 2012/0130137 describes, for example, an oxygen content of 2.5% to 8% by volume in the starting gas.
The need for an oxygen excess for such catalyst systems is common knowledge and is reflected in the process conditions when catalysts of this kind are used. Representative examples include the comparatively recent studies by Jung et al. (Catal. Surv. Asia 2009, 13, 78-93; DOI 10.1007/s10563-009-9069-5 and Applied Catalysis A: General 2007, 317, 244-249; DOI 10.1016/j.apcata.2006.10.021).
JP-A 2011-006381 to Mitsubishi addresses the risk of peroxide formation in the workup section of a process for preparing conjugated alkadienes. As a solution, the addition of polymerization inhibitors to the absorption solutions for the process gases and the setting of a maximum peroxide content of 100 ppm by weight by heating the absorption solutions is described. However, there is no information as to avoidance or monitoring of peroxides in upstream process steps. A particularly critical aspect is the step of cooling the ODH reactor output with a water quench. Organic peroxides formed are barely soluble in water, and so they are deposited and can accumulate in the apparatus in solid or liquid form, instead of being discharged with the aqueous purge stream. At the same time, the temperature of the water quench is not so high that sufficiently high and constant breakdown of the peroxides formed can be assumed.
The catalytic oxidative dehydrogenation can form high-boiling secondary components, for example maleic anhydride, phthalic anhydride, benzaldehyde, benzoic acid, ethylbenzene, styrene, fluorenone, anthraquinone and others. Deposits of these components can lead to blockages and to a rise in the pressure drop in the reactor or beyond the reactor in the workup area, and can thus disrupt regulated operation. Deposits of the high-boiling secondary components mentioned can also impair the function of heat exchangers or damage moving apparatuses such as compressors. Steam-volatile compounds such as fluorenone can get through a quench apparatus operated with water and precipitate beyond it in the gas discharge lines. In principle, there is therefore also the risk that solid deposits will get into downstream apparatus parts, for example compressors, and cause damage there.
US 2012/0130137 A1 paragraph [0122] also refers to the problem of high-boiling by-products. Particular mention is made of phthalic anhydride, anthraquinone and fluorenone, which are said to be present typically in concentrations of 0.001% to 0.10% by volume in the product gas. US 2012/0130137 A1 paragraphs [0124]-[0126] recommends cooling the hot reactor discharge gases directly, by contact with a cooling liquid (quench tower), at first to typically 5-100° C. The cooling liquids mentioned are water or aqueous alkali solutions. There is explicit mention of the problem of blockages in the quench by high boilers from the product gas or by polymerization products of high-boiling by-products from the product gas, and for this reason it is said to be advantageous that high-boiling by-products are entrained as little as possible from the reaction section to the cooling section (quench).
JP-A 2011-001341 describes a two-stage cooling operation for a process for oxidative dehydrogenation of alkenes to conjugated alkadienes. This involves first cooling the product discharge gas from the oxidative dehydrogenation to a temperature between 300 and 221° C. and then cooling it further to a temperature between 99 and 21° C. Paragraphs [0066] ff. state that the temperature between 300 and 221° C. is preferably established using heat exchangers, but a portion of the high boilers could also precipitate out of the product gas in these heat exchangers. JP-A 2011-001341 therefore describes occasional washing of deposits out of the heat exchangers with organic or aqueous solvents. Solvents described are, for example, aromatic hydrocarbons such as toluene or xylene, or an alkaline aqueous solvent, for example the aqueous solution of sodium hydroxide. In order to avoid excessive frequency of interruption of the process to clean the heat exchanger, JP-A 2011-001341 describes a setup having two heat exchangers arranged in parallel, which are each alternately operated or rinsed (called NB operation mode).
JP-A 2013-119530 describes a quench in which an ODH product gas is cooled by direct contact with water. Paragraph 7 addresses the problem that the product gas entrains solid constituents and that these can prevent stable operation. Solid constituents were even said to be found in the offgas of the quench column. Paragraph 41 asserts that these constituents consist mainly of isophthalic acid and terephthalic acid. Even if the amount in the offgas is small, it is said that filters, for example, could be covered very rapidly. According to this application, the solid constituents are eliminated as far as possible from the product gas through suitable choice of internals and of the volume flow ratio of coolant and gas stream. However, the application does not give any information as to how blockage of the coolant circuit can be avoided.
JP-A 2013-177380 describes, in paragraph 60, possible coolants used in the product gas quench. Cooling liquids mentioned in general terms are saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, esters, ethers, aldehydes, ketones, amines, acids, water and mixtures thereof. The preferred coolant is water. Paragraph 62 describes the supply and removal of water as coolant: according to this, at least a portion of the water which has been discharged from the bottom of the cooling tower can be fed back to a middle stage and/or to the top of the cooling tower. The water withdrawn from the bottom may comprise solids. For the removal thereof, the document suggests standard processes, for example the use of a screen. Paragraphs 63 and 64 mention, as by-products which condense out in the coolant, oxygenous organic compounds such as aldehydes, ketones, carboxylic acid, unsaturated aldehydes, unsaturated carboxylic acid, and polymers having the compounds mentioned as a structural unit.
According to WO 2012/157495, the aqueous solution of an organic amine is used as coolant in the product gas quench of an oxydehydrogenation. Paragraph 6 describes the problem of blockage of lines by solids. Accordingly, it has been found that high-boiling by-products such as organic acids, aldehydes and ketones condense when the reaction product gas is quenched with cooling water and flow along with the flow of the reaction product gas, which results in blockage of lines and endangerment of the continuous operation of the plant.
Effective removal of the components is said to be achieved through use of an aqueous solution of an organic amine and of a preferably aromatic solvent. However, the two coolants are used in separate regions of the cooling tower. Thus, paragraph 35 states that a first quench tower is used for the scrubbing of the reaction product gas with the aqueous solution of organic amine, and a second quench tower for the purification of the reaction product gas with the aromatic solvent. Paragraph 38 says that the spent aqueous solution of the organic amine and the spent aromatic solvent can be incinerated.
KR 2013-0036467 and KR 2013-0036468 describe the use of a mixture of water and a water-miscible organic solvent as coolant in a product gas quench of an oxydehydrogenation. Owing to water miscibility, the workup and regeneration of the organic solvent is very energy-intensive and is disadvantageous from an economic point of view.
U.S. Pat. Nos. 3,965,126, 4,219,388 and 4,961,827 describe the removal and purification of maleic anhydride from maleic acid-containing scrubbing solutions. Such scrubbing solutions are obtained, for example, in the production of phthalic anhydride. The recovery of maleic anhydride from phthalic anhydride processes is described, for example, in the PERP Report 2013 May “Maleic anhydride” from Nexant (published December 2013) in chapter 3.5.